1. Field of the Invention
This invention relates to power supply circuits and, more particularly, to direct current voltage converters for providing direct current operating voltages from a direct current voltage source at a different voltage level.
2. Description of the Prior Art
It is common, particularly at remote locations, to generate operating voltages for electronic equipment by converting a locally available electrical supply to the proper current and voltage levels. In telephone carrier systems, for example, remote terminals, as well as intermediate repeaters, are often powered over the telephone line. One such system is shown in the copending application of B. S. Bosik, Ser. No. 974,381, filed Dec. 29, 1978, now U.S. Pat. No. 4,196,321.
In applications such as that described above, power for operating the power supply is made available as a high voltage direct current on the transmission line. Such transmission lines, however, are often exposed to the elements and subject to large voltage surges due to crosses with commercial power lines or to lightning strikes. Such power surges can damage the electronic components of the power converter itself, and can be transmitted through the power converter to damage the electronic circuits operated thereby.
It has been common to utilize gas tubes or other breakdown devices which, under the influence of such high voltages, break down and provide an easy conduction shunt path around the delicate electronic components. In many applications, however, such breakdown devices are not sufficiently rapid acting to prevent harm. Such power supplies, for example, normally utilize a transistor switch to interrupt the current flow through a transformer which provides the voltage conversion. This transistor switch is particularly susceptible to voltage surge damage because of its direct connection in the supply voltage path.